The delivery of radio frequency (RF) energy to target regions within tissue is known for a variety of purposes of particular interest to the present invention. In one particular application, RF energy may be delivered to diseased regions (e.g., tumors) for the purpose of ablating predictable volumes of tissue with minimal patient trauma. RF ablation of tumors is currently performed using one of two core technologies.
The first technology uses a single needle electrode, which when attached to a RF generator, emits RF energy from the exposed, uninsulated portion of the electrode. This energy translates into ion agitation, which is converted into heat and induces cellular death via coagulation necrosis. In theory, RF ablation can be used to precisely sculpt the volume of necrosis to match the extent of the tumor. By varying the power output and the type of electrical waveform, it is theoretically possible to control the extent of heating, and thus, the resulting ablation. The diameter of tissue coagulation from a single electrode, however, is limited by heat dispersion.
The second technology utilizes multiple needle electrodes, which have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. U.S. Pat. No. 6,379,353 discloses such a probe. The ablation probe disclosed in U.S. Pat. No. 6,379,353, referred to as the LeVeen Needle Electrode, comprises a cannula having a needle electrode array, which is reciprocatably mounted within the cannula to alternately deploy the electrode array from the cannula and retract electrode array within the cannula. The individual electrodes within the array have spring memory, so that they assume a radially outward, arcuate configuration as they are deployed from the cannula. In general, a multiple electrode array creates a larger lesion than that created by a single needle electrode.
When creating lesions using needle electrode arrays, RF energy is commonly delivered to the tissue in one of several ways. In the first arrangement illustrated in FIG. 1, RF current may be delivered to an electrode array 10 in a monopolar fashion, which means that current will pass from the electrode array 10 to a dispersive electrode 12 attached externally to the patient, e.g., using a contact pad placed on the patient's flank. In a second arrangement illustrated in FIG. 2, the RF current is delivered to an electrode array 20 in a bipolar fashion, which means that current will pass between “positive” and “negative” electrodes 22 within the array 22. Bipolar arrangements, which require the RF energy to traverse through a relatively small amount of tissue between the tightly spaced electrodes, are more efficient than monopolar arrangements, which require the RF energy to traverse through the thickness of the patient's body. As a result, bipolar electrode arrays generally create larger and/or more efficient lesions than monopolar electrode arrays. To provide even larger lesions, it is known to operate two electrode arrays in a bipolar arrangement. For example, FIG. 3 illustrates two electrode arrays 30 and 32 that are configured to emit RF energy between each other. Specifically, the first electrode array 30 is operated as an active electrode array that emits RF energy, and the second electrode array 32 is operated as a return electrode array that receives the RF energy, thereby ablating the tissue between the electrode arrays 30 and 32.
Physician feedback has indicated that there is a continuing need for treating larger tissue volume. For the electrode configuration illustrated in FIG. 3, the distance between the two electrode arrays affects the volume of tissue ablated. For example, if the distance between the electrode arrays were to be lengthened to try to ablate a longer tissue volume, the energy transmitted between the electrode arrays may thin and not fully ablate the intermediate tissue. As a result, an hour-glass shaped ablation, rather than the desired uniform circular/elliptical ablation, would be created.
As a consequence, when ablating lesions that are larger than the capability of the above-mentioned devices, the common practice is to stack ablations (i.e., perform multiple ablations) within a given area. This requires multiple electrode placements and ablations facilitated by the use of ultrasound imaging to visualize the electrode in relation to the target tissue. Because of the echogenic cloud created by the ablated tissue, however, this process often becomes difficult to accurately perform. This considerably increases treatment duration and requires significant skill for meticulous precision of multiple electrode placement.